In recent years, utility companies operating steam generating units powered by pulverized coal have experienced a variety of problems relating to fuel supplies. Curtailment of the availability of natural gas and oil as fuels; decline in coal reserves due to environmental restrictions on mining and to increased foreign purchases; increased costs of spot purchases in the coal market; and related factors have played a part. Moreover, environmental requirements for low sulfur-dioxide emissions have increased the demand for bituminous coals of low .[.surfur.]. .Iadd.sulfur .Iaddend.content.
One approach to solving some of these problems has been to turn to the use of more economical coals such as the high moisture (as much as 31% total moisture), low sulfur, sub-bituminous coals found in various parts of the United States. Assuming that the moisture content of such coals can be reduced sufficiently to permit conventional pulverizing techniques to be employed, the fuels are attractive to utility companies. The removal of all free moisture and most of the inherent moisture from the coal prior to conventional pulverizing would reduce .[.to.]. .Iadd.the .Iaddend.capacity requirements of the .[.crushing.]. .Iadd.pulverizing .Iaddend.mill by eliminating the adverse effects of surface moisture during pulverizing and by reducing the weight of solids per hour to be pulverized.
A prior art attempt to combine crushing and drying of high moisture content coal is described in a paper presented to the American Power Conference of Chicago, Ill. on Apr. 21 to 23, 1975 entitled .Iadd.Using High Moisture Western Coals in Power Boilers Designed for Pulverized Bituminous Coal Firing .Iaddend.by Robert L. Thiede and W. C. Rogers. Although such prior art systems have achieved a measure of success, difficulties have been experienced due to fires in the incoming air ducting caused by coal particles tossed back up the air ducting from the crusher .[.mill.].. This sort of "foul balling" in the air duct apparently has been due to the inlet geometry of such prior art machines in which the coal and high .[.termperature.]. .Iadd.temperature .Iaddend.air are permitted to mix at a location well above the crusher itself, which can cause undesirable excess turbulence at the crusher inlet plus permit pieces to be thrown back into the air inlet ducting.